1. Field of the Invention
The present invention relates to the field of laser diodes, and more particularly, to thermo-electric cooling circuitry for laser diodes.
2. Description of the Related Art
A data communications network is the interconnection of two or more communicating entities (i.e., data sources and/or sinks) over one or more data links. A data communications network allows communication between multiple communicating entities over one or more data communications links. High bandwidth applications supported by these networks include streaming video, streaming audio, and large aggregations of voice traffic. In the future, these demands are certain to increase. To meet such demands, an increasingly popular alternative is the use of lightwave communications carried over fiber optic cables. The use of lightwave communications provides several benefits, including high bandwidth, ease of installation, and capacity for future growth.
The synchronous optical network (SONET) protocol is among those protocols designed to employ an optical infrastructure and is widely employed in voice and data communications networks. SONET is a physical transmission vehicle capable of transmission speeds in the multi-gigabit range, and is defined by a set of electrical as well as optical standards. A similar standard to SONET is the Synchronous Digital Hierarchy (SDH) which is the optical fiber standard predominantly used in Europe. There are only minor differences between the two standards. Accordingly, hereinafter any reference to the term SONET refers to both SDH and SONET networks, unless otherwise noted.
Lightwave communication requires lasers. There are several types of laser diode configurations used in lightwave communications, including cooled and uncooled laser diodes. Cooled lasers provide better performance, however, cooled lasers further require a thermo-electric cooler (TEC). Uncooled lasers do not require a TEC, but generally perform less efficiently. Cooled laser diodes are typically used for Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Muliplexing (DWDM) applications which increase data throughput of fiberoptic systems. WDM and DWDM example data is shown in Table 1, below. The bandwidths shown are exemplary only and depend on the signal carried. Note that the tighter the spacing the less bandwidth will be available for use.
DWDM systems are typically referred to as those WDM systems for which the channel count is 32 and above. Table 1 shows the spacing of representative channels in frequency units and wavelength units. As shown, the denser spacing allows many more channels. The table also shows that WDM and DWDM systems have narrow bandwidths. The laser diodes used by WDM and DWDM systems are tunable to access the appropriate portions of a communication spectrun. More particularly, the laser diodes used for DWDM applications must be tunable for wavelength channel spacings of as small as 0.8 and 0.4 nanometers as shown in Table 1. The wavelength of the laser diodes are typically tuned by changing the temperature of the laser diodes. Accordingly, systems employing laser diodes for DWDM applications closely monitor the temperature of laser diodes used in DWDM applications to prevent channel interference and crossover noise.
In contrast with DWDM applications, Time Division Multiplexing (TDM) applications do not require close monitoring of laser diodes because the modulation is in the time domain. More specifically, the concerns of channel interference are not an issue for TDM applications because in TDM a particular time slot is assigned to each signal source, and the complete signal is constructed from portions of the signal collected from each time slot, all on the same channel.
Typically, cooled systems for lasers using TEC circuitry can dissipate as much as 10 Watts of energy. Those in the art have attempted to improve the power/performance ratio for typical laser diode systems by attempting to improve the performance of uncooled lasers and by increasing the efficiency of TEC circuitry for cooled lasers with little success. Accordingly, when high performance is required in applications such as lightwave conmmunications, cooled lasers are typically used. Conversely, when lower performance is acceptable for lightwave conmmunications, uncooled lasers are typically used.
What is needed, therefore, is a system and circuitry for improving the power/performance ratio for both cooled and uncooled laser diodes for both high and low performance applications.
In accordance with the present invention, a thermo-electric cooler circuitry and a method therefore provides a cooled laser diode configured to run in either a low power mode or a standard mode. A method for a thermo-electric cooler includes coupling the thermo-electric cooler to a laser diode, operating the thermo-electric cooler in one of a low power mode and a standard mode, the laser diode configured to transmit signals in the low power mode and the standard mode, and switching between the low power mode and the standard mode. The low power mode maintains the laser diode at a temperature within a predetermined range of temperatures. The standard mode maintains the laser diode at a temperature that corresponds to a predetermined wavelength of light output from the laser diode. In general, the wavelength of light output corresponds proportionately to a change in temperature from 0.1 nanometers per degree Centigrade. In one embodiment, the low power mode is a Time Division Multiplexing (TDM) mode and the standard mode is a Dense Wavelength Divison Multipexing (DWDM) mode.
In one embodiment, the predetermined range of temperatures is a range of temperatures within which the laser diode has a user-defined power versus performance ratio and the predetermined range of temperatures are input by one of a user and a system generated source. In another embodiment, the predetermined range of temperatures is determined by a user setting a temperature measure above and below a fixed temperature that corresponds to a wavelength of light output from the laser diode. The temperature measure is either a delta about a fixed temperature or a minimum temperature and a maximum temperature set above and below the fixed temperature.
Another method in accordance with an embodiment relates to a method for providing thermo-electric cooled system. The method includes providing a first mode and a second mode of operating a laser diode wherein the choice of mode is a function of a power and performance requirement. The function, in one embodiment, is a ratio of power versus performance wherein the power required to cool a laser diode is compared with the performance required for one of a plurality of laser diode applications.
Another embodiment of the invention is directed to an optical transceiver including a temperature circuit, a thermo-electric cooler coupled to the temperature circuit, and a laser diode coupled to the thermo-electric cooler. The thermo-electric cooler is responsive to inputs from the temperature circuit, the inputs identifying one of at least a first mode and a second mode, wherein a choice of mode is a function of a performance requirement. In an embodiment, the optical transceiver includes a temperature circuit that includes a switch configured to alter the thermo-electric cooler between the first mode and the second mode.
In an embodiment, the optical transceiver further includes a coupler coupled to the laser diode, the coupler producing an optical signal, an optical fiber coupled to the coupler, and a wavelength signal circuit coupled to the lens and the temperature circuit, the wavelength signal circuit configured to transmit feedback to the temperature circuit to maintain a stable wavelength of the laser diode. In one embodiment the optical transceiver is disposed on an OC-transceiver line card of a synchronous optical network (SONET) communication system.
In an embodiment, the first mode is a low-power mode and the second mode is a standard mode wherein the first mode is configured to permit a predetermined amount of wavelength drift. The first mode is further allows a thermo-electric cooler to dissipate less than 2 Watts depending on the amount of wavelength drift allowed.
Another embodiment is directed to an optical transceiver that includes a circuit for providing a laser diode circuit with at least a first mode and a second mode, wherein a choice of mode is a function of a performance requirement. The performance requirement is one of a high performance requirement including dense-wavelength-division-multiplexing type applications and a low performance requirement including time-domain-multiplexing type applications. The optical transceiver further includes a switch, the switch configured to alter a thermo-electric cooled circuit between the first mode and the second mode. The first mode is a time-division multiplexed (TDM) mode wherein the laser diode is set to a range of temperatures. More specifically, in an embodiment, the (TDM) mode includes a temperature delta wherein the TEC circuit permits temperature variations within the range of temperatures. In one embodiment, the TDM mode operates with no power under normal operating conditions and operates with less than 2 Watts of power in failure conditions, such as air conditioning failures in central office storage facilities. Further, the TDM mode permits wavelength drift while maintaining the performance requirement.